The recent literature abounds in examples of the key role played by miRNAs in determining cell fate. Their fundamental importance is particularly well-defined with regard to cancer emergence, progression, and response to therapy (Gabriely, et al., 2011; Hurst, et al., 2009; Iorio and Croce, 2012; Ma, et al., 2007; Nicoloso, et al., 2009).
Non-coding microRNAs represent potentially valuable new biomarkers for breast cancer detection, staging, and therapy assessment. There are several lines of evidence to suggest that 1) miRNAs are upregulated in a wide range of cancers and can act as tumor suppressors or oncogenes1-6, 2) modulation of miRNA activity has been linked to tumor outcome7-13, and 3) specific miRNA signatures have been linked to the tissue type, differentiation state and developmental lineage14-19. In addition, the combination of standard disease biomarkers with biomarkers derived from non-coding (nc) RNA expression signatures can deliver more comprehensive information about cancer risk assessment and prognosis20-25. The development of nanosensors that are capable of noninvasively identifying miRNA signatures characteristic of metastatic cancers such as metastatic breast cancer would have significant value diagnostically. The only reports of in vivo miRNA detection have recently emerged from a group in Korea, which uses molecular beacon technology37,38. These latest studies support the feasibility of imaging miRNA availability in vivo. Still, the molecular beacon technology employed in these studies suffers from low signal-to-background ratios, making quantitative interpretation of signal intensity problematic. There is a need for increased sensitivity in the detection. Besides these studies, the currently established methods for microRNA detection in situ rely on PCR and northern blotting, or high-affinity hybridization probes39-51. However, these methods are only applicable in vitro. Consequently, these methods do not permit longitudinal studies, in which the “evolution” of the metastatic phenotype is monitored in an intact physiologic environment. Also, such methods, because they rely on hybridization, report on miRNA availability but not on miRNA activity. Further, the existing methods rely on direct hybridization of the sensor oligo to the miRNA, reflecting a 1:1 ratio of fluorescent probe per miRNA, limiting the sensitivity of detection of a small amount in a cell.